1. Field of the Invention
This invention relates to a method and apparatus for controlling and stabilizing the operating wavelength of a laser operating at different output power levels and, more particularly, for controlling and stabilizing the operating wavelength for a semiconductor laser.
2. Description of the Related Art
Optical transmission systems rely heavily on the use of lasers in the transmission network. Optical signals output by lasers are modulated directly or by external modulation to carry information and control signals in optical networks. As the information signals traverse the optical network, they are amplified by Raman techniques or by discrete amplification techniques that rely on pump lasers to stimulate the amplification process. In all these applications, control of laser operating parameters such as output power and operating wavelength is necessary. As the lasers age or undergo environmental changes such as temperature changes, control of the operating wavelength becomes even more critical. Control and stabilization of the wavelength and power of the laser becomes even more difficult when the target values for these parameters are expected to assume one of a multiplicity of values in wide respective ranges.
Various techniques have been developed to stabilize or control the laser operating wavelength, that is, the center emission wavelength of the laser. These techniques include the use of gratings or a wavelength locker device. Gratings can be internal to the laser cavity such as in distributed feedback (DFB) lasers or distributed Bragg reflector (DBR) lasers. Gratings are also written on optical fibers to form a device known as a fiber Bragg grating. Wavelength lockers can operate internal or external to the laser cavity and generally provide wavelength control and monitoring for tunable lasers. Typically, wavelength lockers provide a relative reference for tuning the operating wavelength. No one of these techniques is applicable to all the specific situations that can occur in a transmission system.
DFB and DBR lasers generally provide an optical output spectrum that exhibits a very narrow linewidth. High power transmission for such a narrow linewidth laser is severely hampered because the stimulated Brillouin scattering (SBS) threshold limits the power spectral density that can be transmitted by an optical fiber before SBS deteriorates signal transmission in the fiber. Dithering of the operating wavelength is the technique resorted to in order to avoid or ameliorate the effects of SBS. But this technique is inapplicable to pump lasers used for Raman amplification because it would have the deleterious effect of amplitude modulating the gain in direct response to the dithering applied to the pump laser. As a result, the optical transmission signal would be degraded.
Lasers stabilized by the use of fiber Bragg gratings exhibit a high relative intensity noise (RIN) which limits the suitability of such stabilized lasers as a co-propagating pump lasers in Raman amplification applications. In addition, proper operation of the fiber Bragg grating stabilization technique requires a relatively high laser output power. In a low laser output power environment, the fiber Bragg grating stabilization technique cannot lock the center emission wavelength of the laser over to the desired wavelength designated by the fiber Bragg grating.
Wavelength lockers are relatively expensive and decrease the available power budget because of insertion and device losses. These devices, especially state of the art devices, are only operable with single mode lasers. But lasers that are used as pump lasers in optical amplifier applications tend not to be single mode lasers. Moreover, wavelength locked lasers are operable over a very limited output power range. When a wavelength locked signal laser is initialized in a wavelength division multiplexed (WDM) system, the injection current to the laser is changed to achieve the desired output power. Injection current changes cause a concomitant detuning in the operating wavelength of the laser, which can be sufficiently large to cause interference in the adjacent WDM channels.
None of the techniques known in the prior art provide for practical, inexpensive, and accurate wavelength stabilization and control in a wide range of laser applications and for a broad range of laser output powers, especially where that range includes low output power.